A conventional computing device (e.g., smart phone, tablet computer, etc.) may include a system on chip (SOC), which has a processor and other operational circuits. Specifically, an SOC in a smart phone may include a processor chip within a package, where the package is mounted on a printed circuit board (PCB) internally to the phone. The phone includes an external housing and a display, such as a liquid crystal display (LCD). A human user when using the phone physically touches the external housing and the display.
As the SOC operates, it generates heat. In one example, the SOC within a smart phone may reach temperatures of 80° C.-100° C. Furthermore, conventional smart phones do not include fans to dissipate heat. During use, such as when a human user is watching a video on a smart phone, the SOC generates heat, and the heat is spread through the internal portions of the phone to the outside surface of the phone.
The outside surface of the phone is sometimes referred to as the “skin.” The outside surface includes the part of the external housing that is physically on the outside of the phone as well as any other externally-exposed portions, such as an LCD display. It is generally accepted that the skin of the phone should not reach temperatures higher than about 40° C.-45° C. due to safety and ergonomic reasons. As noted above, the SOC within the smart phone may reach temperatures of 80° C.-100° C., although the temperature of the SOC is not felt directly at the skin of the phone. Instead, heat dissipation within the phone often means that the skin temperature of the phone is at a lower temperature than the SOC temperature. Furthermore, whereas changes to SOC temperature may be relatively quick (e.g., seconds), changes to device skin temperature may be relatively slow (e.g., tens of seconds or minutes).
Conventional smart phones include algorithms to control both the SOC temperature and the skin temperature by reducing a frequency of operation of the SOC when a temperature sensor on the SOC reaches a threshold level. Additionally, the physical properties of a smart phone model and of an individual smart phone itself affect the thermal performance of a smart phone. For instance, a smart phone with a thin form factor is generally expected to experience high skin temperatures more quickly than would a smart phone having a thick form factor. In an additional example, a smart phone model having an air gap or heat spreader between its processor and its skin would generally be expected to experience high skin temperatures more slowly than would a smart phone not having an air gap or heat spreader. Also, manufacturing imperfections and defects may affect the thermal performance of a given smart phone by affecting thermal resistance and heat paths between a smart phone processor and its skin.